The growth of spines from Purkinje cells and basket cells under normal developmental conditions is being studied in Golgi impregnated material and by EM serial reconstruction in mice. Some aspects of the maturation of neurons in brain aggregates reconstructed in vitro from dissociated cells of chick embryos telencephalon will be studied by the Golgi impregnation of these aggregates. A Silver-chromate technique has been successfully developed in this laboratory for this purpose.